Oncogenic retroviruses are important agents in the etiology of cancer and other diseases in humans and animals. Current descriptive studies of human T-cell lymphotropic virus (HTLV)-I/-II epidemiology are defining the modes of HTLV-II transmission and its occurrence in Africa and Asia, but no clear cut disease association has been found. In the etiology of non-Hodgkin's lymphoma (NHL) the attributable risk for HTLV-I, especially in those under age 50 is 40-60%. HTLV-I antibody-negative individuals exposed to HTLV-I have increased cell mediated response compared to those who are not exposed, and low income and high HTLV-I maternal titer are independent risk factors for seroconversion in children of HTLV-I seropositive mothers. Studies of human immunodeficiency virus type 1 (HIV-1) have documented a growing list of risk factors and associated malignancies. The Multistate AIDS and Cancer Registry has demonstrated increased risk for anal and cervical cancer in people who are HIV positive (84x for anal cancer and 5x for cervical cancer), documented trends in Kaposi's sarcoma and non-Hodgkin's lymphoma and evaluated other possible cancer associations. A controlled intervention trial is underway in Malawi cleansing the birth canal and neonate with chlorhexidine, an antiseptic known to neutralize HIV and other infectious agents to reduce transmission of HIV to infants from their HIV-positive mothers. HLA or closely linked genes have a significant effect on controlling HIV infection. Rates of disease progression are concordant in hemophiliac HIV-infected brothers who share 1 or 2 HLA haplotypes, but not those who share zero HLA haplotypes. Incidence of HIV-1 in Trinidad with an HIV subgroup B, the same strain circulating in the U.S.A, is high making this site a prime locale for comprehensive interventions, including an HIV vaccine trial. Helicobacter pylori may play an important role in non-EBV-associated gastric carcinoma. A multinational pilot case-control study of nasopharyngeal carcinoma has been completed in Taiwan.